Combined air intake and combustion gas vent terminals, sometimes referred to as vent/intake terminals, have long been used with fuel-fired heating appliances, particularly side wall-vented gas fireplaces and furnaces. Combined vent/intake terminals typically comprise concentrically mounted vent and intake conduits, with a larger intake conduit disposed around a smaller vent conduit. The terminal is installed in an exterior wall of a building, with the intake and vent openings exterior to the building. The recent popularity of side wall-vented furnaces, also known as horizontally-vented or direct-vented furnaces, is due to the ease with which the required air intake and flue systems may be installed in a building. Correspondingly, there has been an increased demand for vent/intake terminals because such devices simplify installation effort and cost, as only a single fixture need be installed.
A problem commonly encountered with vent/intake terminals, however, is an unwanted recirculation of combustion gases into the terminal intake, which reduces the efficiency of combustion in the fuel-fired appliance. Such recirculation is principally caused by the close proximity of the intake and vent openings. Prior art vent/intake terminals typically have attempted to minimize recirculation by placing an anti-mixing baffle or separator between the intake and vent openings. The anti-mixing baffle is usually a baffle member disposed around the vent outlet to block direct axial access to the intake inlet, thereby impeding direct recirculation of axially vented combustion gases into the intake inlet of the terminal. Intake air enters the terminal radially, between the baffle and the building wall, then turning 90.degree. to travel axially though the intake conduit of the terminal. Thus, although the terminal conduits are concentric, the baffle causes the radial flow of intake air to be physically separated from the axial flow of vented combustion gases, thereby impeding mixing.
These prior art terminals, however, have disadvantages. One well-known disadvantage results from wind-induced pressure effects on the operation of the terminal. The differing orientations of the intake and vent openings in the prior art terminals result in a wind-induced pressure differential between the openings. For example, a direct axial wind would blow directly on the axially-oriented vent outlet, but the radially-oriented intake opening would be shielded by the anti-mixing baffle, creating a dynamic pressure differential. A wind-induced pressure differential is undesirable because it modifies the pressure differential generated by the appliance between the intake and vent openings, hereinafter referred to as the combustion pressure differential. The combustion pressure differential, which causes intake air to be induced into the appliance and combustion gas expelled therefrom, is carefully balanced in high-efficiency furnaces to permit an efficient combustion of fuel in the appliance. Wind effects at the terminal, however, unbalance the combustion pressure differential between the intake and vent, adversely affecting the efficiency and/or operation of the appliance.
One solution to the wind pressure problem experienced with prior art terminals is suggested in U.S. Pat. No. 5,282,456 to Smelcer. This patent discloses a combined vent/intake terminal, for use with an induced draft, natural gas furnace, which has axially-aligned and unimpeded intake and vent openings. This avoids the wind-blocking effect of the anti-mixing baffle of the prior art. To impede mixing without using an anti-mixing baffle, the vent opening of the '456 terminal is extended axially away from the wall, and away from the intake opening, to increase the distance between the intake openings and the vent openings. To further impede mixing, the '456 terminal relies on the draft inducer fan of an induced draft furnace to forcibly eject combustion gases through the vent opening, thereby projecting combustion gases further away from the wall and from the intake opening.
The design of the '456 terminal, however, also has disadvantages. Foremost among these is the terminal's reliance on a reasonably high pressure draft inducer fan to forcibly eject combustion gases from the terminal, thereby limiting mixing of intake and combustion gases. Furthermore, the speed and pressure at which the draft inducer fan operates reduces the sensitivity of the vent/intake system to wind-induced pressure imbalances across the intake inlet and vent outlet.
Today, oil-fired furnaces are becoming more popular. These furnaces are sometimes referred to as a positive pressure furnace because the combustion draft inducing means responsible for generating the combustion pressure differential is located on the intake side of the furnace rather than the vent side, contrary to induced draft gas furnaces. The combustion pressure differential generated in a positive pressure furnace, however, is significantly smaller than that generated by the draft inducer fan of an induced draft gas furnace, and in some cases may be only one tenth as much. A reduced combustion pressure differential causes positive pressure furnaces to be more sensitive to wind effects than induced draft gas furnaces. Furthermore, the reduced combustion pressure differential results in lower vent gas speeds at the vent outlet, making a concentric vent/intake terminal used therewith more susceptible to permitting recirculation of vent gases.
The combined intake/vent terminal of the '456 patent, when used with a positive pressure furnace, has limited wind pressure balancing characteristics. While the pressure imbalances permitted by the '456 terminal are overcome by the draft inducer fan of an induced draft furnace, the '456 terminal is significantly less effective when used with a positive pressure furnace. Furthermore, the '456 terminal permits an unacceptable amount of mixing of intake air and combustion gas due to the lower speed at which vent gases are expelled from a positive pressure furnace, especially when a direct frontal wind is encountered. Accordingly, there is a need for a intake/vent terminal with improved pressure-balancing and anti-recirculation characteristics.
Combined vent/intake terminals are also susceptible to partial or complete blockage of the intake and/or vent openings, by snow, ice or other environmental debris. If furnace combustion is permitted to continue when the intake and/or vent openings are blocked, excessive smoking can result, endangering the safe and efficient operation of the appliance. Accordingly, the vent and intake pressures should be monitored by the furnace control system and, if a blockage is sensed, the operation of the burner ceased.
Prior art safety shut down systems of this nature, however, have relied on pressure sensors and pressure switches located in, or immediately adjacent, the fuel fired appliance itself. These have the disadvantage that the pressure sensing systems must be calibrated to compensate for various firing rates, and for pressure effects of aerodynamic friction occurring in the furnace intake/vent piping system used, such effects depending on the size, length and configuration of the intake and venting conduits. Accordingly, there is a need for an improved safety shutdown system for detecting furnace vent and intake blockage.